1. Field of the Invention
The present invention relates to a thin-film capacitor and a method of producing the same and more particularly, to a thin-film capacitor using a "perovskite-structured" oxide film as its dielectric intervening between a pair of electrodes, which is suitably used for Large-Scale Integrated circuits (LSIs), and a method of producing the capacitor.
2. Description of the Prior Art
To realize the next-generation 1-Gigabit (Gbit) or greater Random-Access Memories (DRAMs), conventionally, various research an development have been made for practically utilizing perovskite-structured oxides, such as (Ba,Sr)TiO.sub.3 (i.e., BST), (Pb,Zr)TiO.sub.3 (i.e., PZT), and so on, as a capacitor dielectric of the storage capacitors. This is because perovskite-structured oxides are excellent in dielectric characteristic, insulation performance, and chemical stability.
Sputtering, Chemical Vapor Deposition (CVD), and sol-gel methods have been known as typical methods of making a perovskite-structured oxide thin-film. To realize Gbit-class DRAMs, the well-known "stacked-capacitor" structure needs to be used for their storage cells and at the same time, the sidewalls of the stacked capacitors need to be effectively utilized to obtain the capacitance as high as possible. Thus, CVD methods having an excellent step-coverage property are preferred for this purpose. From this point of view, there has been the strong need to establish the film-formation method of perovskite-structured oxides using CVD in the field of LSIs.
It was reported by E. Fujii et al. that perovskite-structured polycrystalline oxide thin-films formed by CVD had a columnar grain structure, in other words, they are formed by columnar crystal grains, in the treatise entitled "Preparation of PbTiO.sub.3 thin films by plasma-enhanced metalorganic chemical vapor deposition", Applied Physics Letters, Vol. 65, No. 3, pp. 365-367, Jul. 18, 1994.
In general, to apply perovskite-structured polycrystalline oxide thin-films to Gbit-class DRAMs, it is required for the thin-films to have a small thickness of approximately 30 nm or less for the purpose of decreasing the equivalent silicon-dioxide (SiO.sub.2) thickness t.sub.eq. With the perovskite-structured polycrystalline oxide thin-films having the columnar grain structure, however, comparatively large irregularity or roughness exists on the film surface, which varies dependent upon the size of the columnar grains. The surface irregularity or roughness of the perovskite-structured oxide thin-film leads to the roughness of the interface between the dielectric and the top electrode of the capacitor. As a result, when a voltage is applied across the top and bottom electrodes of the capacitor, electric-field concentration tends to occur at the interface of the dielectric and the top electrode, causing some problems, such as increase in leakage current, degradation in dielectric breakdown characteristic, and so on.
On the other hand, it was reported by T. Kawahara et al. that the current leakage was improved by the use of BST thin-films formed by a two-step deposition process (i.e., two-step thermal CVD processes) in the treatise entitled "Surface Morphologies and Electrical Properties of (Ba,Sr)TiO.sub.3 Films Prepared by Two-Step Deposition of Liquid Source Chemical Vapor Deposition", Japan Journal of Applied Physics, Vol. 34, pp. 5077-5082, Part 1, No. 9B, September 1995.
With the two-step deposition process disclosed in the treatise by Kawahara et al., the BST film is formed through the steps shown in FIG. 1. Specifically, a bottom electrode is formed in the step S101 and then, a first BST film of approximately 5 nm in thickness is formed on the bottom electrode at a deposition temperature of 420.degree. C. in the step S102. The first BST film is then subjected to heat treatment in the step S103, thereby crystallizing the first BST film. Subsequently, a second BST film is formed on the first BST film at a same deposition temperature of 420.degree. C. in the step S104 so that the first and second BST films have a total thickness of 30 nm. The second BST film is then subjected to heat treatment in the step S105, thereby crystallizing the second BST film. Finally, a top electrode is formed on the second BST film in the step S106. Thus, a thin-film capacitor is completed.
It is seen from the treatise by Kawahara et al. that the resultant surface flatness of the second BST film is improved and accordingly, the equivalent SiO.sub.2 thickness t.sub.eq is 0.56 nm and the leakage current density is 1.2.times.10.sup.-8 A/cm.sup.2 at an applied voltage of 1.1 V.
To apply perovskite-structured polycrystalline oxide thin-films to capacitor dielectrics of Gbit-class DRAMs, however, it is necessary to suppress the oxidation reaction of the bottom electrode occurring at the interface of the bottom electrode and the capacitor dielectric during the growth process of a perovskite-structured oxide. This is because an oxide is formed at the interface due to oxidation of the surface of the bottom electrode to serve another capacitor dielectric together with the perovskite-structured oxide, arising a problem that the effective or resultant dielectric constant of the capacitor dielectric is lowered.
Also, it is necessary to suppress the oxidation reaction of a silicon part or a silicon substrate located below the bottom electrode during the growth process of a perovskite-structured oxide. For example, if a polysilicon plug interconnecting the bottom electrode with a diffusion region formed in the silicon substrate is provided, the polysilicon plug tends to be oxidized at the interface of the bottom electrode and the plug. Thus, a problem that contact resistance of the plug with the bottom electrode is raised will occur.
As a result, it is required that the growth temperature of a perovskite-structured oxide is as low as possible (e.g., lower than 420.degree. C.).
Moreover, from the viewpoint of productivity, the two-step deposition disclosed by T. Kawahara et al. is not preferred because of the increased number of necessary process steps. It is required to realize a simple method of making a perovskite-structured oxide.